1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to an apparatus having a particulate filter (hereinafter referred to, as “DPF”) which traps particulates (particulate matter) in exhaust gases of the internal combustion engine.
2. Description of the Related Art
Japanese Patent Laid-open No. 2005-307880 (JP'880) discloses an exhaust gas purifying apparatus having a DPF and a pressure difference sensor for detecting a pressure difference between an upstream side pressure and a downstream side pressure of the DPF. JP'880 further discloses a method for determining an abnormality in the pressure difference sensor. Specifically, an estimated value of an amount of particulates accumulated in the DPF (operating condition dependent estimated accumulation amount PMsm) is calculated according to an engine operating condition. An upper limit value and a lower limit vale of the pressure difference are calculated according to the operating condition dependent estimated accumulation amount PMsm, and the pressure difference detected by the pressure difference sensor is compared with the calculated upper and lower limit values. An abnormality of the pressure difference sensor is determined according to the comparison result.
The pressure difference sensor disclosed by JP'880 is connected to the upstream and downstream sides of the DPF in the exhaust pipe through an upstream side passage and a downstream side passage, respectively, which are provided to help facilitate the pressure detection. Even when an abnormality, such as a leak (disengagement) or clogging, has occurred in the downstream side passage, the pressure difference detected by the pressure difference sensor does not significantly differ from the pressure difference detected when the downstream side passage is normal. Therefore, it is difficult to detect an abnormality in the pressure difference sensor using the method sensor and method taught by JP'880, especially when an exhaust gas flow rate is relatively small.